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Substantial reduction of NOX and particulate emissions from diesel engines will be required by the emission legislation in the future. In a diesel engine, the combustion and emissions formation are governed by the spray formation and mixing processes. Parameters of importance are droplet size, droplet distribution, injection velocity, in-cylinder flow (convection and turbulence) and cylinder charge temperature/pressure. The mixing is controlled by convective and turbulent mixing due to in-cylinder charge motion, momentum transfer and turbulence induced by the injection process. The most important processes are known to be the turbulent macro- and micromixing. Smaller nozzle orifices are believed to increase mixing rate, due to smaller droplet size leading to faster evaporation. Dimensional analysis suggests that the turbulent mixing time, τmix, scales with orifice diameter, d.